As the rotary head type digital audio tape recorder of the type above-mentioned (hereinafter referred to as the R-DAT), there is known, for example, an audio tape recorder as shown in "Technology of Television--Serial No. 427", a monthly magazine published by Denshi Gijyutsu Syuppan Co., Ltd., in Apr., 1987, Pages 109 to 117 and in "Electronics Life", a monthly magazine published by Nippon Housou Shuppan Kyoukai in Mar. 1987, Pages 11 to 66.
According to such a tape recorder, a pair of rotary magnetic heads each having plus or minus azimuth are disposed at the rotary drum (head cylinder), as separated from each other by 180.degree. on the periphery of the rotary drum. The rotary magnetic heads helically scan a magnetic tape travelling as wound on the drum at 90.degree. . At the time of recording, the heads successively generate, on the tape in a guard bandless manner, tracks A and B in which digital audio signals (PCM audio signals), tracking control ATF signals and the like have been recorded in an area division format shown in FIG. 8.
Each of the tracks A and B has each of the azimuths of the pair of heads. The tracks are overlappingly recorded and generated such that each track has a width equal to 2/3 of the width of each head.
Each of the tracks A and B consists of 196 blocks, each having a 288-bit signal length. From the trace starting end, each track is successively time-divided into a first sub code area SUB1, a first ATF area ATF1, a PCM area PCM, a second ATF area ATF2 and a second sub code area SUB2.
PCM audio signals are recorded in the PCM area PCM, and search signals, time signals and the like relating to the PCM audio signals are recorded, as sub information, in the first and second sub code areas SUB1 and SUB2. ATF signals required for tracking control at the time of reproduction, such as synchronizing signals and pilot signals, are recorded in the first ATF area ATF1 at the starting end and the second ATF area ATF2 at the terminal end, in a format of the 4-track completion type (to be discussed later).
Each of the first ATF area ATF1 and the second ATF area ATF 2 has a five-block length. When a synchronizing signal of a trace track is detected, the following signals are recorded, as the ATF signals, in time division in a pattern repeated at the period of four tracks in which the recording length and the recording order are changed;
(i) a tracking pilot signal having a single frequency f.sub.1 (130.67 KHz);
(ii) a synchronizing signal as a sampling reference signal of which the frequency successively varies between f.sub.2 and f.sub.3 for every track; and
(iii) an erasion signal as a margin having a single frequency f.sub.4 (=1.568 MHz).
These signals are recorded such that the crosstalk components of the pilot signals from adjacent two tracks are sampled.
The frequency of the synchronizing signal is f.sub.2 =522.67 KHZ for the track A having a so-called plus azimuth, and f.sub.3= 784.00 KHz for the track B having a minus azimuth.
As to the recording length of each signal, five types of 0.5 .tau. 1.tau., 1.5 .tau., 2 .tau., and 2.5 .tau. are available where the length of one block is defined as 1 .tau..
The following description will discuss, in detail, the format of the four-track completion type with reference to five continuous tracks B , A.sub.1, B.sub.1, A.sub.2 and B.sub.2 in FIG. 9 which shows, in an enlargement scale, the first ATF area ATF1 and the second ATF area ATF2.
It is now supposed that an f.sub.3 synchronizing signal, an erasion signal and a pilot signal are successively recorded in the first track B.sub.0 at the area ATF1 thereof from the trace starting end, these signals respectively having recording lengths of 1.tau., 2 .tau.; and 2 .tau.. It is also supposed that a pilot signal, an erasion signal, an f.sub.3 synchronizing signal and an erasion signal are successively recorded in the first track B.sub.0 at the area ATF2 thereof from the trace starting end, these signals respectively having recording lengths of 2 .tau., 1 .tau., 1.tau. and 1.tau..
In this case, a pilot signal, an erasion signal, an f.sub.2 synchronizing signal and an erasion signal are successively recorded in the second track A.sub.1 at the area ATF1 thereof from the trace starting end, these signals respectively having recording lengths of 2 .tau., 1 .tau., 0.5 .tau. and 1.5 .tau.. An f.sub.2 synchronizing signal, an erasion signal and a pilot signal are successively recorded in the second track A.sub.1 at the area ATF2 thereof from the trace starting end, these signals respectively having recording lengths of 0.5 .tau., 2.5 .tau. and 2 .tau..
An f.sub.3 synchronizing signal, an erasion signal and a pilot signal are successively recorded in the third track B.sub.1 at the area ATF1 thereof from the trace starting end, these signals respectively having recording lengths of 0.5 .tau., 2.5 .tau.and 2 .tau.. A pilot signal, an erasion signal, an f.sub.3 synchronizing signal and an erasion signal are successively recorded in the third track B.sub.1 at the area ATF2 thereof from the trace starting end, these signals respectively having recording lengths of 2 .tau., 1 .tau., 0.5 .tau.and 1.5 .tau..
A pilot signal, an erasion signal, an f.sub.2 synchronizing signal and an erasion signal are successively recorded in the fourth track A.sub.2 at the area ATF1 thereof from the trace starting end, these signals respectively having recording lengths of 2 .tau., 1 .tau., 1 .tau.and 1 .tau.. An f.sub.2 synchronizing signal, an erasion signal and a pilot signal are successively recorded in the fourth track A.sub.2 at the area ATF2 thereof from the trace starting end, these signals respectively having recording lengths of 1 .tau., 2 .tau.and 2.tau..
As to the fifth track B.sub.2, the areas ATF1 and ATF2 thereof have the same recording patterns to those of the areas ATF1 and ATF2 of the first track B.sub.0, respectively.
A margin gap IBG having a length of 3 .tau.is disposed between the area ATF1 and the PCM area PCM and between the area ATF2 and the PCM area PCM.
At the time of reproduction, a head trace is controlled by tracking control discussed in the following.
The head gap width is 1.5 time the width of each of the tracks A and B. Accordingly, a reproduced signal from each head generally includes a signal component of the trace track and crosstalk components from two tracks adjacent to the trace track.
The levels of both crosstalk components vary with the trace position. When the head center coincides with the track center to provide an on-track position, the levels of both crosstalk components are equal to each other.
It is apparent from FIG. 9 that, when a normal reproduction is made in which a tape T travels in the same direction as that at the recording time such that both areas ATF1 and ATF2 of the tracks A and B are scanned, the pilot signal of the right-hand track i.e., the succeeding track is crosstalked while that portion of the trace track in which the synchronizing signal has been recorded is scanned. After such scanning, the pilot signal of the left-hand track, i.e., the preceding track is crosstalked.
Based on a head changeover pulse signal (RFSW signal) of which level is inverted for every half-turn of the head cylinder;
(i) the next trace head may be identified; and
(ii) the frequency of the synchronizing signal of each of the ATF areas ATF1 and ATF2 of the next trace track may be estimated.
At the time of reproduction, the crosstalk components of the pilot signals of adjacent two tracks are sampled and extracted based on:
(i) the identification of the trace head by the RF pulse signal;
(ii) the estimation of the frequency of each synchronizing signal of the trace track; and
(iii) the detection of the synchronizing signal of each of the ATF areas ATF1 and ATF2 of the trace track.
A difference in level between both crosstalk components is then operated, thereby to generate a tracking control signal, i.e., a tracking error signal which is proportional to the level difference.
The level of the tracking error signal varies with a shift in trace position between both heads. Accordingly, based on the tracking error signal, the tape T is controlled in travelling phase such that both heads are brought to the on-track position, thereby to control the head traces.
In the R-DAT, a tape is normally recorded and reproduced with the tape travelling speed and the drum revolution speed respectively set to the standard travelling speed 8,150 mm/sec. and the standard revolution speed 2,000 r.p.m in the standard mode. However, only in a long-time mode called Option 2, the tape may be recorded in a double period of time with the tape travelling speed and the drum revolution speed respectively lowered to 4,075 mm/sec. i.e., a half of the standard travelling speed and to 1,000 r.p.m. i.e., a half of the standard revolution speed.
The tape recorded in the long-time mode may be reproduced with the tape travelling speed set to 4,075 mm/sec, i.e., the same travelling speed as that used at the recording time. According to the standards of the R-DAT, the drum revolution speed at the reproduction time is to be also set to the speed determined for the long-time mode used at the time of recording.
However, when the tape is reproduced in the longtime mode with the use of the same travelling speed and the same revolution speed as those used at the recording time, the tape/head relative speed is lowered to 1/2 of that in the standard mode and the frequency of the reproduced signal from each head is also lowered to 1/2 of that in the standard mode.
Accordingly, if the characteristics of the electromagnetic conversion system such as the heads, a rotary transformer and the like, as well as the characteristics of the reproduction circuit system such as a reproduction equalizer circuit and the like, are set based on the signal characteristics in the standard mode, an electromagnetic conversion processing, a reproduction equalizer processing and the like may not be executed in the best conditions at the time of reproduction in the long-time mode.
Therefore, when the drum revolution speed at the time of reproduction in the long-time mode is to be set to the revolution speed determined for the long-time mode, it is required to switch the characteristics of the electromagnetic conversion system, the characteristics of the reproduction equalizer and the like, from the characteristics in the standard mode. This disadvantageously presents problems of complicating the entire arrangement and the like.
In view of the foregoing, there is proposed, for example, an arrangement as shown in Japanese Utility Model Laid-Open Publication 55205/1987 (G11B 5/09) in which, at the time of reproduction in the long-time mode, the drum revolution speed is changed to a speed double the revolution speed predetermined for the longtime mode, i.e., the standard revolution speed, such that reproduction is carried out without changing the characteristics of the electromagnetic conversion system, the reproduction circuit system and the like.
In this case, the drum revolution speed at the reproduction time is double the speed used at the recording time. Accordingly, each head scans the tape at a speed double the speed determined for the long-time mode, and the tape is repeatedly scanned substantially at the same position thereof by both heads. That is, the tape is reproduced by a so-called double scanning method.
It is therefore difficult to achieve tracking control using the above-mentioned ATF signals. Accordingly, in the system disclosed by the above-mentioned Publication, the tracking control is not made, but based on the detection of an error in a reproduced signal, a reproduction processing is made by selecting a reproduced PCM audio signal and a PCM signal as sub information which are less erroneous.
However, when reproduction is made without tracking control, the trace position of each head deviates from the on-track position from time to time due to tracking shift. This causes the head reproduction level to be lowered to considerably deteriorate the error rate in each reproduced signal. This fails to achieve a good reproduction. Further, if a state in which the error rate is deteriorated, continuously takes place, this creates the problem that reproduction cannot even be made.